Interview

Could Evaporation Be a Significant Source of Renewable Energy?

One drawback of wind and solar power is intermittency, resulting in the need for energy storage. In a Yale Environment 360 interview, biophysicist Ozgur Sahin explains how evaporation from lakes and reservoirs could be transformed into a stable source of renewable energy.

In 2015, biophysicist Ozgur Sahin found a way to turn the process of evaporation into a renewable source of energy, building devices that lit up LEDs and electrified a toy car — all powered by changes in humidity levels in the air. Now, Sahin and colleagues at Columbia University have published a new paper in the journal Nature Communications that estimates evaporation on U.S. lakes and reservoirs could theoretically supply nearly 70 percent of the country’s electricity needs.

Ozgur Sahin

The technology, which uses water-absorbing spores from the soil bacterium B. subtilis to generate energy, is still in the early stages of development. But Sahin, an associate professor of biological sciences and physics at Columbia and a recipient of a 2016 Young Investigator Award from the Office of Naval Research, says it has important advantages over other forms of renewable energy. “Wind and solar power are intermittent sources,” he said. “In the case of evaporation, you do not fluctuate very much. You can expect to have very significant evaporation even at night.”

In an interview with Yale Environment 360, Sahin talks about how this new kind of electricity generation works, its limitations, and the need for broad scientific collaboration to test whether the technology could feasibly become a significant source of power in the U.S. and globally.

Yale Environment 360: In work published in 2015, you and colleagues invented a couple of machines that used the process of evaporation to generate mechanical force. You did this by utilizing harmless bacterial spores that expand and contract in response to changes in humidity. One of the devices you call the “evaporation engine.” Describe how that particular machine works.

Sahin: The evaporation engine has spore-coated plastic strips. The spores expand and contract in response to changes in humidity, and they do that with a lot of strength. And so when that happens, these strips elongate and shorten in response. They’re essentially working like a muscle, which you can then convert into electrical energy.

Humidity itself typically doesn’t change so rapidly in the environment — it changes on a daily timescale — but when you have an open water surface, you have evaporation and you can harness that with a device. The way we do that is we put the strips below shutters. When they open up, they can let moisture go through, and when these shutters are closed they block moisture. You can connect that [device] to a generator or something that converts movement and mechanical energy into electricity, and thereby you can generate electricity. This is the general concept behind our device.

e360: The second machine you devised is called the “moisture mill.” You were able to set a toy car in motion with that one. It looks like a wheel and works on the same principle. Tell me a little about that.

Sahin: This second device basically has a plastic circle and spore-coated plastic strips that are placed around the circle. We insert the circle into a chamber halfway, and inside the chamber it’s humid because there’s a wet paper that’s lining the walls of the chamber, and that humidity causes spores on that half of the circle to expand. And on the outside, the remaining half of the circle, the air is dry, so strips on that side are a little more curly because the spores are in a contracted state.

When that happens, the center of gravity of the circle becomes a little offset from the rotation axis of the circle. The [subsequent] rotation brings spores that were in the dry half, into the humid region, and it takes some humid spores that are in the humid part, that go into the dry part. It keeps on rotating as long as the paper inside the chamber is wet. We put together a simple, toy-like system out of Lego parts that starts to roll on the surface. It’s not the fastest car, but it’s probably the first one that’s running off of water.

e360: In this latest paper, you explore the potential of natural evaporation as a renewable energy source. You envision open water sources such as lakes and reservoirs covered by evaporation-driven energy harvesters. What would this look like? Would there simply be bigger versions of that device with the opening and closing shutters on top of these open water sources?

Sahin: Some basic concepts would probably be used, but you don’t necessarily need the same shutter mechanisms, which would be a complicated system on a large scale. You would have to come up with something that would function, and that would have few moving components, to make it easier to implement at large scale.

e360: Do you envision using the bacterial spores, or are there other ways to harness evaporation power?

Sahin: One goal is to make something like a sheet out of the spore material, so these sheets can expand and contract on a surface area, and that in itself could be the basis of the device. The final material could be producing a significant amount of power. Maybe not at the theoretical limits we predicted, but it might still be attractive when you consider costs and other factors.

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e360: You calculate that in the U.S. up to 325 gigawatts of evaporation power is potentially available, which, as you write, is close to 70 percent of the electrical energy generation rate in the U.S. in 2015. How did you arrive at that figure?

Sahin: What we included in that number are the existing reservoirs and lakes known to us. We ignored reservoirs that are too small, and we also ignored the Great Lakes. We did that because it’s easier to model open bodies of water when they are not so big — “not so big” meaning if they’re smaller than 100 kilometers across.

The first thing we assumed is that the device can work at a level within the limits of thermodynamics. Then we wanted to find out, how would the environmental conditions impose further limits? So, weather will affect evaporation rates, temperature, wind speed, how much sunlight is available at that particular location, the humidity of the air. We put all those parameters into a model, something we actually built by using previous models developed by hydrologists … From there we calculated in a given location with given weather data, what would be the power you can get from a specific area — say, a square meter? That basically gives us a map of the country, and you can say that if there’s a body of water at this location, you can expect this much power from that water body.

“Even a small fraction of this available potential, if it becomes realized, would still be significant.”

e360: Were you surprised at the total figure?

Sahin: The number in and of itself is big, but if this technology gets built, it would definitely be smaller. But that can still make a contribution to renewable energy because existing renewable energies are not developed at that scale, on the order of tens of gigawatts. Even a small fraction of this available potential, if it becomes realized, would still be significant.

e360: What are the advantages or disadvantages that evaporation energy might have compared to solar or wind?

Sahin: Wind and solar power are intermittent sources. You need energy storage technologies to address that because consumers need power at different times of the day and different times of the year. In the case of evaporation, you basically do not fluctuate very much. For example, you can expect to have very significant evaporation even at night. The main reason for that is that water itself can store heat. So, the heat that comes from sunlight during the day gets stored in water, and it stays there at night, and it can power evaporation directly. So that creates a smooth profile of evaporation rates. Then, if you have devices, you gain additional control over the evaporation rate. So that control, together with the storage ability of water, is a potential way to match power demand to power supply without a reliance on batteries or other energy-storage mechanisms.

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The other benefit is the direct relationship to water resources. Harvesting energy from evaporation necessarily reduces the evaporation rate. So if you have artificial reservoirs — for example, those created for irrigation or hydropower — reducing that water loss could be another important benefit. That water could be used by farmers or by other habitats downstream.

In terms of disadvantages, many of these water bodies have recreational and other uses, such as for fishing. There could be a compromise, if it comes to that. It might also be possible, envisioning materials that are flexible like sheets made out of some biological material, that these things can potentially be removed and placed back with more ease than rigid constructions like solar farms and wind turbines. Of course, this has to be developed and tested, but the way I see it, this can potentially be in harmony with both the environment and other potential users of these resources.

e360: Of course, there may be other negative environmental impacts.

Sahin: Absolutely. What we, as researchers, are hoping for is to get the attention of more people and experts in other fields who can think about either potential problems, or solutions to these potential problems. Energy projects themselves are very complicated, and they have many different aspects, from economics to the environment. It’s hard to undertake such a project in one research laboratory, so what we’re trying to achieve with this publication is to present one piece of the puzzle: There is significant potential for power. Now it’s worth looking into these other aspects that might make this either a possibility or challenge its implementation. But even if there are challenges, there might be ways around them because it appears to be a relatively flexible technology.

“What we should do is go beyond the proof-of-principle device and see how the technology might look and what it might lead to.”

e360: Do you have any concerns regarding the pitfalls that lay ahead in terms of developing this technology?

Sahin: There could be things I didn’t anticipate, and I think right now what we should do is basically make demonstrations and go beyond the proof-of-principle device and see how the technology might look and what it might lead to. That could inform future discussions.

e360: In your paper you have a table in which you give a state-by-state breakdown of potential power generation. Utah, California, and Texas come in as the top three. Aside from available open water surface area, what are the environmental conditions that make for greater potential power generation from evaporation?

Sahin: The more a location gets sunshine, the more power you can expect from a given area. And if the area is dry, that is something that enhances evaporation rates. That also increases the power from a given area, and those two components tend to be larger in the southwest United States.

e360: Your study was restricted to the U.S., but do you imagine that evaporation energy might have applicability in off-grid areas of developing countries?

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Sahin: We limited our studies to the U.S., primarily because we had access to the data that go into our models. But in general we expect this to be applicable in many other parts of the world. If a farmer has a pool of water used for irrigation, that could be utilized as a source of energy.

e360: What are some next steps for your lab?

Sahin: We’re mainly focusing on getting from these individual spores to large materials by assembling spores into sheets. Using that, we will try to build larger devices, not necessarily something in the size of a pilot test, but maybe a bigger tabletop device, maybe a small pool that we can assemble and test on the surface of that water. And that would probably inform us on whether the materials are ready for a larger test. But there are still a lot of basic questions that need to be answered.

Diane Toomey is an award-winning public radio journalist who has worked at Marketplace, the World Vision Report, and Living on Earth, where she was the science editor. Her reporting has won numerous awards, including the American Institute of Biological Sciences' Media Award. She is a regular contributor to Yale e360 and currently is an associate researcher at the PBS science show NOVA. More about Diane Toomey →